The M21 is a designated marksman rifle designed during the Vietnam war, a response to a United States Army request for an accurate sniper rifle. The M14 was selected as a base for its accuracy, reliability and semi-automatic capability.

In 1969 the Rock Island Arsenal converted 1,435 National Match M14s by adding an optical sight and Match grade ammunition, these rifles were then designated XM21.

An improved version with a fiberglass stock was designated as the M21 in 1975, and remained the Army's primary sniper rifle until 1988 where it was replaced by the M24 Sniper Weapon System. Although the M21 is still in service with certain units of the US Military.